A photoelectrophoretic imaging composition containing b-carotene

ABSTRACT

There is disclosed a photoelectrophoretic imaging system wherein the imaging suspension utilized contains a photomigratory pigment dispersed in an insulating carrier liquid. The imaging suspension contains a vitamin precursor, beta carotene, in conjunction with the other components of the vehicle. The suspension is interpositioned between at least two electrodes and subjected to a potential difference while substantially simultaneously being selectively exposed to a reproducible image by a source of electromagnetic radiation. As a result of the suspension of the present invention it is now possible by regulating the polarities of the electrodes in the system to produce both negative and positive images from a single sense input.

United tates Paieni [72] inventor Raymond L. .ieiio Sodas Point, NY. 2:Appl. No. 887,878 [22] Filed Dec. 24, 1969 [45] Patented Oct. 26, 1971[73] Assignee Xerox Comoration Rochester, NY. Continuation-impart ofapplication Ser. No. 624,005, May 29, 1967, now abandoned.

[54] A PHOTOELECTROPHORETKC KMAGHNG COMPOSITION CONTAENHNG B-CAROTENE 3Claims, 1 Drawing Fig.

[52] 11.8. C1 96/88, 96/].3, 96/1.5,117/37 LE, 204/181 [51 int. Ci..G03g 13/22, 003g 5/06 [50] Field oiSearch 96/1, 1.3, 1.5, 88; 260/66C; 204/181; 1 17/37 LX; 252/501 [5 6] References Cited UNITED STATESPATENTS 3,384,488 5/1968 Tulagin et a1. 252/501 X 3,384,565 5/1968Tulagin et al. 96/13 X 3,384.566 5/1968 Clark 204/181 OTHER REFERENCESCherry et al., Photoconductivity of Carotenoids," 1967, Vol. 3,Molecular Crystals, pp. 251- 259 Primary Examiner-George F. LesmesAssistant Examiner-R. E. Martin Allomeys- Paul M. Enlow, James J.Ralabate, David C. Petre and Donald C. Kolasch As a result of thesuspension of the present invention it is now possible by regulating thepolarities of the electrodes in the system to produce both negative andpositive images from a single sense input.

PATENTEUUU 26197! 3,616,398

INVENTOR. RAYMOND L. JELFO A TTORNE) A PEOTOELECTROPH'DRETEC iMAGHNGMPOSITION CONTAINHNG i3AROTENE This application is acontinuation-in-part of US. Pat. application Ser. No. 642,005 filed May29, 1967, now abandoned and relates to an imaging system and morespecifically to a photoelectrophoretic imaging system.

BACKGROUND 0? THE iNVENTiON in photoelectrophoretic imaging coloredphotosensitive particles are suspended in an insulating carrier liquid.This suspension is then placed between at least two electrodes,subjected to a potential difference and exposed to a light image.Ordinarily, in carrying out the process, the suspension is placed on atransparent electrically conductive member in the form of a thin film,and exposure is made through this member while a second generallycylindrically shaped biased electrode is rolled across the suspension.The particles are believed to bear an initial charge when suspended inthe liquid which causes them to be attracted to the transparent baseelectrode and upon exposure, to change polarity so that the exposedparticles migrate to the second or roller electrode thereby formingcomplementary images on each of the electrodes by particle subtraction.The process may be used to produce both poiychromatic and monochromaticimages. in the latter instance a single color photoresponsive particlemay be used in the suspension or a number of differently coloredparticles may be used all of which respond to the exposure radiation. Anextensive and detailed description of the photoelectrophoretic imagingtechniques as described above may be found in US. Pat. Nos. 3,383,993;3,384,488; 3,384,565; and 3,3 84,566, and are hereby incroporated byreference.

Although it has been found that good quality images can be produced,especially when a relatively insulating blocking electrode" surface isused, due to the nature of the process the photoresponsive particlestend to respond in one particular imaging mode. Thus, in a monochromaticsystem, acceptable imaging has been found to be generally restricted toa single sense process so that with positive input a negative image maybe reproduce. in order to obtain a positive output the image input mustbe a negative image.

it is, therefore, an object of this invention to provide an imagingsystem which will overcome the above-noted disadvantages.

it is a further object of this invention to provide a novel imagingsystem capable of a high degree of flexibility.

Another object of this invention is to provide a novel imaging systemcapable of producing high-contrast, backgroundfree images in a one stepprocess.

Yet, still a further object of this invention is to provide a novelphotoelectrophoretic monochromatic imaging system.

Still a further object of the present invention is to provide a novelmonochromatic imaging composition.

Yet, still another object of the present invention is to provide aone-step imaging system capable of producing highcontrast images ofeither optical sense regardless of the sense of the input information.

SUMMARY OF THE iNVENTlON The foregoing objects and others areaccomplished in accordance with this invention, generaliy speaking, byproviding a suspension of photoelectrophoretic imaging particles in aninsulating carrier liquid. The suspension utilized in the process of thepresent invention contains ,B-carotene, a vitamin precursor additive,which makes it possible upon proper orientation of the electrodepolarities to readily produce high contrast, background-free images inboth a positive and negative imaging sense. The suspension isinterpositioned between at least two electrodes and subjected to anelectrical field. The suspension is selectively exposed to areproducible image by a source of electromagnetic radiation. Generallyspeaking, the imaging suspension is placed on a transparent electricallyconductive member of first electrode in the form of a thin film,

and exposure is made through the transparent member while in contactwith a second imaging electrode which is placed or roller over the topof the imaging suspension. The photomigratory particles present in thissuspension respond to the electromagnetic radiation to form a visibleimage pattern at one or both of the electrodes, the images beingcomplementary in nature. The imaging suspension employs intenselycolored pigment particles the photosensitive pigment portion of whichserves both as the colorant and as the photosensitive material.Additional photosensitive materials are not required thus providing avery expedient imaging process.

it has been determined that upon introducing fl-carotene, a vitaminprecursor, into the photomigratory imaging suspension of the presentinvention it is possible, upon proper orien tation of the electrodepolarities, to produce high-contrast, background-free images in either apositive to negative or positive to positive imaging mode. lieretonow,the materials used as components of the imaging suspension havesubstantially restricted the imaging capabilities of thephotoelectrophoretic monochromatic imaging system. it has now beendemonstrated that upon introducing into the imaging suspension theB-carotene additive, it is now possible to produce images of eithersense by changing the polarity of the bias on the electrodes in thesystem.

DESCREPTJION OF THE DRAWINGS The invention is more specifically definedin the accompanying drawing in which there is seen a transparentelectrode generally designated l which, in this instance, is made up ofa layer of optically transparent glass 2 overcoated with a thinoptically transparent layer 3 of tin oxide. Tin oxide coated glass ofthis nature is commercially available under the trade name NESA glass."This electrode shall hereafter be referred to as the injectingelectrode. Coated on the surface of the injecting electrode E is a thinlayer 4 of finely divided photosensitive pigment particles dispersed inan insulating carrier liquid. The term pliotosensitive," for thepurposes of this application, refers to the properties of a particlewhich, once attracted to the injecting electrode, will migrate away fromthis electrode under the influence of an applied electric filed when itis exposed to actinic radiation. A further detailed explanation of theapparent mechanism of the operation is disclosed in the above U.S. Pat.Nos. 3,384,565 and 3,384,566.

The imaging suspension will consist of specifically colored, finelydivided photosensitive particles dispersed in an insulating carrierliquid or vehicle. Any suitable photosensitive pigment particle may beused such as disclosed in US. Pat. Nos. 3,384,565 and 3,384,566. Asabove stated, the pigment portion of the photomigratory particleprovides both the photosensitivity and coloration for the respectiveparticle. Any suitable insulating carrier liquid may be used in thecourse of the present invention. Typical insulating carrier liquidsinclude long chain saturated aliphatic hydrocarbons such as decane,dodecane and tetradecane, kerosene fractions such as Sohio OdorlessSolvents available from Standard Oil Co. of Ohio, lsopar G commerciallyavailable from Humble Oil Co. of New Jersey and paraffin wax, moltenbeeswax and other molten thermoplastic materials, mineral oil, corn oil,linseed oil, olive oil, marine oils such as sperm oil and cod liver oil,silicone oils such as dimethyl polysiloxane (Dow Corning Co.),fluorinated hydrocarbons such as Freon and mixtures thereof. The imagingsuspension may also contain a sensitizer and/or binder for the pigmentparticles. To the imaging suspension of the present invention is added avitamin precursor ingredient, beta carotene, generally in amountsranging from about 0.0025 to about 1.0 gram per gram of photoresponsiveor photomigratory pigment. Preferred amounts fall in the range of fromabout 0.0125 to 0.5 grams with op timum results being obtained in arange of from about 0.02 to about 0.1 gram.

Above the liquid suspension is passed a second or imaging electrode 5which, in this illustration is represented as a roller having aconductive central core 11 connected to a power source 6. The core inthis instance is covered with a layer 12 of blocking material which may,for example, be polyurethane, capable of blocking DC current andreferred to as a blocking layer. A transfer sheet 13 or ordinary bondpaper is attached to the outer surface of the roller. The blocking orimaging electrode is connected to one side of potential source 6 throughswitch 7. The opposite side of potential source 6 is connected to theinjecting electrode 1 so that when switch 7 is closed an electric filedis applied across the liquid suspension 4 between electrode 1 and 5. Thepigment suspension is exposed by way of the projector mechanism made upof a light source 8, a transparency 9, and a lens system 10. Forpurposes of this illustration a microfilm positive is used during theprocess. A detailed description of the types of materials which may beemployed as the blocking layer may be found in U.S. Pat. No. 3,383,993.The blocking electrode 5, having a cylindrical configuration in thepresent illustration, is rolled across the top surface of the injectingelectrode 1 supporting the suspension 4 containing the photornigratoryparticles. Switch 7 is closed during the period of image exposure. Uponproper orientation of the electrode polarities, the light exposurecauses the exposed pigment particles suspended in the carrier to migrateto the surface of electrode leaving behind a complementary image on thesurface of the injecting electrode 1 of the unexposed particles. Uponreversing the polarities at which the filed is applied the image senseis reversed while maintaining the same input information. It should benoted at this point of the discussion that although the blockingelectrode in the present illustration is represented as a cylinder itmay also take the form of a flat-plate electrode, as in the case of theillustrated injecting electrode, and the blocking electrode could be theoptically transparent electrode and exposure made through it. Thus, itis to be understood that it is not intended that the structuralarrangement of the apparatus represented by the illustration berestricted to the design as set out herein and all similarconfigurations which will satisfy the requirements of the presentinvention are contemplated. For example, all electrodes utilized may becylindrically shaped thus providing for an expedient continuous process.

The pigment image produced need not necessarily be formed on the surfaceof an electrode but may in fact be formed on a removable paper substrateor sleeve superimposed on or wrapped around the blocking electrode orotherwise interpositioned between the electrodes at the site of imaging.The pigment image may then be fixed in place as for example by placing alamination over its top surface such as by spraying with a thermoplasticcomposition, or by solvent evaporation. The image may also betransferred to the surface of a receiver substrate to which it may inturn be fixed. This would especially be desirable in the case where theimage is formed directly on the electrode surface. Such a transfer stepmay be carried out by adhesive pickoff techniques or preferably byelectrostatic filed transfer while the image is still wet. The blockinglayer itself may be in the form of a removable sleeve in which instanceit is simply replaced following imaging with a similar material. Whenthe image is formed on a substrate wrapped about or superimposed on theelectrode itself it is only necessary to disengage the substrate fromthe electrode surface. The system herein described produces ahigh-contrast monochromatic image with little or no backgrounddegradation in either a positive or negative or positive to positiveimaging mode by a simple reversing of the polarities within the system.

Any suitable material may be used as the receiving or transfer substratefor the image produced such as paper or various transparent plasticssuch as Mylar (polyethylene terephthalatc), Tedlar (polyvinylfluoride)or cellulose acetate sheets, the latter particularly if it is desirableto produce a transparency suitable for image projection.

When used in the course of the present invention, the term injectingelectrode should be understood to mean that it is an electrode whichwill preferably be capable of exchanging charge with the photosensitiveparticles of the imaging suspension hen the suspension is exposed tolight so as to allow for a net change in the charge polarity on theparticle. By the term blocking electrode is meant one which issubstantially incapable of injecting charge carriers into the abovementioned photosensitive particles thus substantially blocking DCcurrent. The use of the blocking electrode serves to minimize particleoscillation in the system.

it is preferred that the injecting electrode be composed of an opticallytransparent material, such as glass, overcoated with a conductivematerial such as tin oxide, copper, copper iodide, gold or the like;however, other suitable materials including many semiconductivematerials such as raw cel-,

lophane, which are ordinarily not thought of as being conductors butwhich are still capable of accepting injected charge carriers of theproper polarity under the influence of an applied electric field may beused within the course of the present invention. The use of moreconductive materials allows for cleaner charge separation and preventsposible charge buildup on the respective electrode, the latter tendingto diminish the interior electrode field. The blocking layer of theimaging electrode, on the other hand, is selected so as to prevent orgreatly retard the injection of charge carrier into the photosensitivepigment particles when the particles reach the surface of thiselectrode. Although a blocking electrode material need not necessarilybe used in the system, the use of such a layer is preferred because ofthe markedly improved results which it is capable of producing. It ispreferred that the blocking layer, when used, be either an insulator ora semiconductor which will not allow for the passage of sufficientcharge carriers, under the influence of the applied field, to dischargethe particles finely bound to its surface thereby preventing particleoscillation in the system. The result is enchanced image density andresolution. Even if the blocking layer does allow for the passage ofsome charge carriers to the photosensitive particles it still will beconsidered to fall within the class of preferred materials if it doesnot allow for the passage of sufficient charge so as to recharge theparticles to the opposite polarity. Exemplary of the preferred blockingmaterials used are baryta paper, Tedlar, Mylar and polyurethane. Anyother suitable materials having a resistivity of from about 10 ohmcm. orgreater may be employed. Typical materials in this resistivity rangeinclude cellulose acetate coated papers, cellophane, polystyrene andpolytetrafluoroethylene. The core of the blocking electrode generallywill consist of a material which is fairly high in electricalconductivity. Typical conductive materials including conductive rubber,and metal foils of steel, aluminum, copper and brass have been foundsuitable. Preferably, the core of the electrode will have a highelectrical conductivity in order to establish the required fielddifferential in the system; however, if a material having a lowconductivity is used a separate electrical connection may be made to theback of the blocking layer of the blocking electrode. For example, theblocking layer or sleeve may be semiconductive polyurethane materialhaving a conductivity of from about 10 to 10" ohms-cm. lf a hard rubbernonconductive core is used then a metal foil may be employed as abacking for the blocking sleeve. Other materials that may be used inconjunction with the injecting and blocking electrodes and otherphotosensitive particles which may be used as the photomigratorypigments and the various conditions under which the process operates maybe found in the above-cited issued patents U.S. Pat. Nos. 3,384,565 and3,384,566 as well as U.S. Pat. Nos. 3,384,488 and 3,383,993.

It is to be understood that any suitable photosensitive pigmen! particlesuch as identified in the above-cited patents may be employed within thecourse of the present invention with the selection depending largelyupon the photosensitivity and the spectral sensitivity desired. Typicalphotoresponsive organic materials include substituted and unsubstitutedorganic pigments such as phthalocyanines, for example, copperphthalocyanine; beta form of metal-free phthalocyanine;tetrachlorophthalocyanine; and x-form of metal-free phthalocyanine;quinacridones as for example 2,9-demethyl quinacridone; 4,1 l-dimethylquinacridone; 3,l0-dichloro-6,l3- dihydro-quinacridone; 2,9-dimethoxy-6,l 3-dihydro-quinacridone and 2,4,9,l ltetrachloro-quinacridone;anthraquinones such as l,5-bis-(betaphenylethylamino) anthraquinone;l,5-bis-( 3'-methoxypropylamine) anthraquinone; l,2,5,6-di-(C,C-diphenyl)-thiazole-anthraquinone; 4-(2'-hydroxyphenyl-methoxyamino)anthraquinone; triazines such as 2,4- diaminotriazine; 2,4-di-( l'-anthraquinonyl-amino)-6-( l pyrenyl)-tria2ine; 32,4,6 tri-( l l",l-pyrenyl)-triazine; azo compounds such as 2,4,6-tris-(N-ethyl-Nhydroxy-ethyl-paminophenylazo) phloroglucinol; l,3,5,7-tetrahydroxy-2,4,6,8-tetra (N-methyl-N-hydroxy-ethyl-p-amino-phenylazo) naphthalene;l,3,S-tri-hydroxy-Z,4,6-tri-( 3 -nitro-N-methyl-N-hydroxy-methyl-4'-aminophenylazo) benzene; metal salts and lakes ofazo dyes such as calcium lake of fi-bromo-l (1'-sulfo-2-naphthylazo)2-naphthol; barium salt of 6-cyano-ll'-sulfo-2-naphthylazo)-2-naphthol; calcium lake of l'(2'-azonaphthaliene-l-sulfonic acid)-2-naphthol; calcium lake ofl-(4'5-chloroazo-benzene-2sulfonic acid)-2-hydroxy- 3-naphthoic acid;and mixtures thereof. Other organic pigments include polyvinylcarbazole;tri-sodium salt of 2-carboxyl phenyl azo (2-naphthiol-3,6-disulfonicacid; N-isopropylcarbazole; 3-benzylidene aminocarbazole;3-aminocarbazole; l-(4-methyl-5-chloro-2'-sulfonic acid) azobenzene-2-hydroxy-3-naphthoic acid; N-2 pyridyl-8, l 3-dioxodinaphtho-( 2,1-b; 2,3-d)furan-6-carboxamide; 2-amino-5- chloro-p-toluene sulfonic acid andthe like.

Typical inorganic photosensitive compositions include cadmium sulfide,cadmium selenide, cadmium sulfoselenide, zinc oxide, zinc sulfide,sulfur, selenium, antimony sulfide, lead oxide lead sulfide, arsenicsulfide, arsenic-selenium, sulfurselenium and mixtures thereof. Theimaging suspension may contain one or more different photosensitiveparticles each having various ranges of spectral response.

A wide range of voltages may be applied between the electrodes in thesystem. For good image resolution, high-image density and low backgroundit is preferred that the potential applied to such as to create anelectric filed of at least about 300 volts per mil across the imagingsuspension. For example, when the imaging suspension is coated to athickness of about 1 mil the electrode spacing will be such that anapplied potential of about 300 volts produces a field across thesuspension of about 300 volts per mil. Potentials as high as 8,000 voltshave been applied to produce images of high quality. As is apparent theapplied potential necessary to obtain the desired filed of strength willvary depending upon the interelectrode gap as well as the type andthickness of the blocking material utilized. The upper limit of thefield strength appears to be limited primarily by the breakdownpotential of the suspension. The imaging suspension is generally coatedto a thickness of up to about I mil or 25 microns, with a preferredoperational thickness being in the range of from about 3-5 microns.

DESCRlPTlON OF THE PREFERRED EMBODiMENTS To further define the specificsof the present invention, the following examples are intended toillustrate but not limit the particulars of the present system. Partsand percentages are by weight unless otherwise indicated.

In the following examples, the injecting first electrode consists ofNESA glass as described above. The imaging or blocking electrodeconsists of a conductive steel core coated with a layer of polyurethane,unless otherwise indicated. A potential of about 7,000 volts is appliedacross the imaging suspension.

EXAMPLE I A commercial, metal-free phthalocyanine is purified by acetoneextraction to remove organic impurities. Since this extraction stepyields the less sensitive beta crystalline form, the desired alpha formis obtained by dissolving lOO grams of the beta form in 600 cc. ofsulfuric acid. precipitating it by pouring the solution into 3,000 cc.of ice water and washing with water to neutrality. The thus purifiedalpha phthalocyanine is then salt milled for 6 days and desalted byslurrying in distilled water, vacuum filtering, water washing, andfinally, methanol washing until the the initial filtrate is clear, thus,producing x-form phthalocyanine. After vacuum drying to remove residualmethanol, the x-form phthalocyanine produced is used to prepare animaging suspension according to the following formulation:

phthalocyanine (x-torm) 10 g. beta carotene (fl) 0.3 3. sperm oil (ADM38 BW) 250 cc. tricresyl phosphate 18 g.

The phthalocyanine is ground in a mortar, placed in a Waring blenderwith the other ingredients and dispersed for about 10 minutes at highspeed. The resulting suspension is coated on the surface of a NESA glasselectrode. As the blocking electrode with paper sleeve is passed acrossthe surface of the suspension liquid at a rate of about 4 inches persecond, the potential is established as stated above and the suspensionis selectively exposed to a light intensity of about [2 foot-candlesthrough a positive transparency with a General Electric visible lightsource. The blocking electrode is maintained as the positive pole andthe NESA glass electrode the negative pole. By a single materialtransfer a high-quality negative image with little or no background isformed on the surface of the paper sleeve.

EXAMPLE ll The process of example I is repeated with the exception thatthe polarities on the blocking electrode and the N ESA glass electrodeare reversed. There results a high-quality positive image on the surfaceof the paper sleeve thereby demonstrating the capability of the presentsystem to image in a direct manner in a positive to positive imagingmode. Comparing the results of example 1 and example ll, there isdemonstrated the claimed capabilities of the present invention, that is,the capability of imaging in either a positive to negative or positiveto positive imaging mode, obtaining images in a single-pass system ofhigh-quality, high-contrast, low-background characteristics.

EXAMPLE [I] An imaging suspension of the following formulation isprepared:

phthalocyanine (x-form) 4 g. olive oil 20 cc. mineral oil 56 cc.tricresyl phosphate 4 3. beta carotene 0.1 g.

The phthalocyanine, prepared according to the process of example I, isground in a mortar, placed in a Waring blender with the otheringredients and dispersed for about 10 minutes at high speed. Theresulting suspension is coated on the surface of the NESA glasselectrode and imaged through a positive transparency as in example lwith the blocking electrode with paper sleeve having a positive polarityand the NESA glass injecting electrode a negative polarity. Theresulting image produced on the paper sleeve is similar to that obtainedin example I.

EXAMPLE IV The process of example Ill is repeated with the exceptionthat the polarities on the two electrodes are reversed. The resultinghigh-quality positive image obtained on the paper sleeve furtherdemonstrates the high-quality positive to positive image capable of thepresent system.

Two separate imaging suspensions are prepared according to the processof example i. However, in the present examples, the beta carotenecomponent is excluded from each formulation. The imaging suspensionprepared is coated on the NESA electrode. The polarity of the blockingelectrode is made positive and the polarity on the injecting electrodenegative. The resulting image produced on the paper sleeve of example Vaccording to the process described in example I is of a highquality,low-background nature. Next, the polarities of the two electrodes arereversed with the blocking electrodes made negative and the injectingelectrode positive. The resulting image produced from this arrangement,example Vi, is of a low-quality, low-contrast nature. in comparing thesetwo examples, that is, examples V and VI, it is evident that theelimination of the beta carotene component from the imaging formulationrestricts the image capability of the system, to a positive to negativeor negative to positive imaging system.

EXAMPLE VII The process of example i is repeated with the exception thatthe following formulation is utilized:

Watchung Red B l g. phthnlocynnine (x-lorm) 4 g. mineral oil 80 cctricresyl phosphate 20 g. beta carotene 0.05 g.

Utilizing a positive transparency at the input end with the blockingelectrode having a negative polarity and the injecting electrode apositive polarity a high-quality positive image is produced on the papersleeve of the blocking electrode.

EXAMPLE VllI The process of example I is repeated with the exceptionthat the following formulation is utilized:

Algol Yellow I g. phthnlocyanine (x-form) 4 g. tricresyl phosphate 2 g.linseed oil I06 cc. styrene 20 g, beta carotene 0.05 g.

With a negative transparency at the input end and with the blockingelectrode having a negative polarity and the injecting electrode apositive polarity, a high-quality negative image is produced on thesurface of the paper sleeve of the blocking electrode.

Although the present examples were specific in terms of conditions andmaterials used, any of the above-listed typical materials may besubstituted when suitable in the above examples with similar results. inaddition to the steps used to carry out the process of the presentinvention, other steps or modifications may be used, if desirable. Forexample, the process may be readily adapted to be used in a continuousimaging system. In addition, other materials may be incorporated in theimaging suspension, injecting electrode or blocking electrode toenhance, synergize or otherwise desirably effect the properties of thissystem for their present use. For example, the imaging suspension maycontain sensitizers for the photoconductive particles which aredissolved or suspended in the carrier liquid.

Anyone skilled in the art will have other modifications occur to himbased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

l. A photoelectrophoretic imaging composition comprising a vitaminprecursor, beta carotene, and a plurality of photoelectrophoreticimaging particles dispersed in an insulatin carrier liquid each of saidparticles comprising an electrica ly photosensitive pigment w lch isboth the primary electrically photosensitive ingredient and the primarycolorant for said particle.

2. The composition as disclosed in claim I wherein said beta carotene ispresent in an amount about 0.0025 to about 1.0 gram per gram of saidphotosensitive pigment.

3. A photoelectrophoretic imaging composition comprising a plurality ofphotoelectrophoretic imaging particles and beta carotene dispersed in aninsulating carrier liquid, each of said particles comprising anelectrically photosensitive pigment which is both the primaryelectrically photosensitive ingredient and the primary colorant for saidparticle, said carrier liquid comprising at least one member of thegroup consisting of mineral oil, olive oil, sperm oil, and linseed oil.

2. The composition as disclosed in claim 1 wherein said beta carotene ispresent in an amount about 0.0025 to about 1.0 gram per gram of saidphotosensitive pigment.
 3. A photoelectrophoretic imaging compositioncomprising a plurality of photoelectrophoretic imaging particles andbeta carotene dispersed in an insulating carrier liquid, each of saidparticles comprising an electrically photosensitive pigment which isboth the primary electrically photosensitive ingredient and the primarycolorant for said particle, said carrier liquid comprising at least onemember of the group consisting of mineral oil, olive oil, sperm oil, andlinseed oil.